I'm going to summarize plot below in the course of writing, so spoilers will abound. you have been cautioned as the dictates of conscience require.

Why call the sections iterations? Do they repeat patterns? Will each section somehow rise from the same conditions? Will the actions repeat themselves in disguise?

This first one is vignetted as Allana mentions below by surreptitious recordings capturing Sybil Gerard at two different moments in her life. We first see her in the novel's future, 1905, in Cherbourg, France (the port opposite England across the Channel) as an old woman. She's watching a trans-channel airship, escorted by small unmanned aeroplanes. It is one of these planes that has recorded her image. Optically encoded, we are told.

The end of the Iteration has a much younger Sybil Gerard in a daguerreotype taken by the Public Morals Section of the Sûreté Générale. Sam Houston is in this image, too, but blurred, we are told, an accidental subject. I think this suggests Sybil is the intentional subject. Who are the Sûreté Générale? The Encyclopedia Britannica says they were an organ of the French Revolutionary government that helped administer the Reign of Terror in the late 1700s. The Committee of General Security.

So, we start with an elderly Sybil, well established in France, but looking toward Britain. There is commerce between the nations, as evidenced by the airship, but there is also tension, suspicion, as evidenced by the surveillant escort. There are unmanned drone planes. There is high resolution optical encoding. There are many things odd about this 1905.

We end with a younger Sybil, on her advent in France, escaped from espionage, politics and murder back in London, in possession of diamonds and a forged identity, and the knowledge of some unique machine punch cards, taking tea in a Paris café. She seems to be the deliberate target of surveillance by the secret service arm of the French government. One suspects this interest must be sparked by something more substantial than her past as a fallen woman.

Are both instances of surveillance, 1855 and 1905, related? Does she remain a person of interest to spies and state powers from the moment she stepped onto the continent well into her final years? A person of interest, but also, somehow, influence, not to be trifled with but still to be spied on from afar? With surreptitious daguerreotypes and optically encoded enhanceable recordings from passing unmanned government aerodrones?

In the course of the first iteration we learn that Sybil is the fallen daughter of a hanged counter-revolutionary leader, who resisted the rise of an Engineered culture. She has long since ceased to be of any political importance, having been used and ruined in a romance with a nobleman named Charles Egremont, who has since become a Member of Parliament. She falls into the life of an escort, and in that life falls in with Dandy Mick Radley.

This picture of Robert Cornelius is the earliest known Daguerreotype from America - I've put it here because this fellow looks like my mental image of Dandy Mick Radley.

Dandy Mick has aspirations, and is playing in powerful circles. He is scheming to fabricate popular support for the restoration of Sam Houston to the Presidency of the Republic of Texas, with the notion that he may then retire on Houston's patronage to an estate in Texas and an advance to a more genteel class. To get there, though, he has waded waist deep in dangerous politics. The kind of politics, as Allana mentions below, that lead to knife blade guttings and the close quarters discharge of small pistols.

This picture of San Houston linked to from the Wikimedia Commons.

By the time Sybil emerges in Paris, having survived the ruin of Mick's enterprise, she has done several things that might have raised her importance in the eyes of state intelligence agencies:

1) She has stolen diamonds from Sam Houston's hollow walking stick and left him bleeding (but not, in the end, dead) on the floor of a hotel in London.

2) She has had her machine-encoded identity record altered to escape her past as a prostitute.

3) She has personally mailed to Paris a box of extraordinary Camphorated cellulose kino cards, which she knows, if they are run on a powerful Engine in Paris, will demonstrate something profound. Something which Mick believed would make his fortune.

4) She has telegraphed an ominous threat to the MP Charles Egremont, suggesting that he and his family might suffer for the betrayal he committed. This telegraph was sent from the lobby of the very hotel in which, a few minutes later, Sam Houston lay bleeding from a bullet that came from Dandy Mick's (her partner in adventure) gun. Dandy Mick as well lay dead from a knifing, and she, alone, escaped. A mysterious Texian assassin committed both atrocities, but escaped unseen. All that any investigating authorities would find would be items and circumstances that tie Mick and Sybil to the attack.

You can see why the government of France might be interested in finding out why she has appeared on the streets of Paris.

As an added layer of significance that I also didn't know about the last time I read this book, all these characters, Sybil Gerard and her father, Dandy Mick and Charles Egremont, all of them are characters from a victorian novel by Benjamin Disraeli called Sybil, or the Two Nations. That link goes to the Project Gutenberg page for the novel, you can get the whole text free online. The Difference Engine is essentially a sequel to the Disraeli novel, picking up with these characters where his novel left off. This is somehow incredibly delightful for me to know.

Finally, I wanted to find a good picture of a man riding a four-wheeled velocipede. Allana has decided this fellow is my cameo in the book, and I'll accept that, because that guy in his stripped jersey, his scarf and his goggles, tooling along the midnight asphalt of the steam powered capitol of the British Empire, strapped into a pedal contraption of his own design, is simply the height of insouciance in the novel so far. Sybil imagined him an inventor of some sort. I like to think he has continued to refine his velocipede, until, several years later, he is the confident master of this:

This picture of Stayer Tomy Hall (1) and his pacer Cissac linked to from letterlust's Flickr photostream.